In a gas turbine, operational efficiency increases as the temperature of the combustion gas stream increases. Higher gas stream temperatures, however, may produce higher levels of nitrogen oxide (NOx), an emission that is subject to both federal and state regulation in the U.S. and subject to similar regulations abroad. A balancing act thus exists between operating the gas turbine in an efficient temperature range while also ensuring that the output of NOx and other types of emissions remain below the mandated levels.
Recent combustion concepts involve the use of a number of nozzles with many small passages in the combustor as opposed to several nozzles with larger passages. These nozzles with small passages offer fast fuel/air mixing in a short flow residence time. The nozzles also provide strong wall heat transfer in combination with effective cooling using fuel and/or air. Thus, these small nozzles or other types of combustion nozzles may have the capability to reduce emissions and also to permit the use of highly reactive types of syngas and other fuels, especially high hydrogen fuels. The design of the nozzles, however, may need to utilize more of the combustor cap space so as to distribute the air properly among the numerous small nozzles.
To minimize the emissions and the potential for flashback, it may be desirable to have as uniform an airflow distribution across the nozzles as possible. Current combustor designs may have nozzle to nozzle or even passage to passage airflow variations therein. The outer most nozzles or tubes may receive less airflow due to a local flow separation as the air approaches the nozzles. Such separation may impact nozzle operability as the nozzles with less airflow may suffer flame holding or flashback. Separation also may impact combustion generated emissions, such as Nitrogen Oxides (NOx) and Carbon Monoxide (CO). The extent of the uneven airflow distribution also may change with load or the total air mass flow rate. In the case of a combustor with a short liner or no liner, the cap surface may be curved so as to let the nozzles flow slightly inward. Such a design, however, may need more air near the outer diameter region then currently may be provided.
There is thus a desire to provide a more uniform airflow distribution about the combustor and the combustor cap. Preferably such a uniform airflow should provide both reduced emissions as well as improving the overall performance of the gas turbine engine, particularly with the use of highly reactive syngas, hydrogen fuels, and similar types of fuels.